1. Field of the Invention
The present invention relates to apparatus for detecting the presence of an underwater object, for example, a submarine, a school of fish, swimmers, an iceberg, an ice layer, polluting materials, vegetation or a thermal current, and may be used to detect an underwater intruding object, for example, a "friendly" vessel or an intruding enemy vessel or target.
2. Description of the Prior Art
The various acoustic, ultrasonic, optical and radio frequency techniques which have previously been proposed or implemented for this purpose have the disadvantage that they are costly, complicated to manufacture, install and operate, or undesirably limited in range and accuracy.